How old was Arjuna when he fought Karna?

Answering this question as per Southern Recension Kumbhakonam Edition because in other versions clear mention of timeline was not found.

Pandu died on Arjun’s 14 th birthday. Immediately they left for Hastinapur.

jagAma hAstinapuraM ShoDashAbdo yudhiShThiraH .. 1\-134\-2 (5935)
bhImasenaH pa~nchadasho bIbhatsurvai chaturdashaH .
trayodashAbdau cha yamau jagmaturnAgasAhvayam .. 1\-134\-3 (5936)

 

Adi Parva,Mahabharat Southern Recension Kumbhakonam Edition

Then they were admitted into Drona’s Gurukul.

Pandavas lived thirteen years at Hastinapur before they went to Lakshagrih.

tatra trayodashAbdAni dhArtarAShTraiH sahoShitAH .
ShaNmAsA~njAtuShagR^ihAnmuktA jAto ghaTotkachaH .. 1\-134\-4 (5937)

 

So of those thirteen years they might have got trained in Gurukul for minimum ten years after which the Exhibition Of skills happened and then Yudhishtira was coronated as Yuvaraja.

So when Arjun faced Karna in Rangabhumi where their first duel happened he must be 24 or 25 years old. Karna might be atleast ten years older to Arjuna by then.

The Fight After Draupadi Swayamvara

This can be considered actual first duel between them.

How old was Arjuna by then?

ShaNmAsA~njAtuShagR^ihAnmuktA jAto ghaTotkachaH .. 1\-134\-4 (5937)
ShaNmAsAnekachakrAyAM varShaM pA~nchAlake gR^ihe .

 

Adi Parva MahaBharat Southern Recension

Pandavas lived in Lakshagrih for six months

Then they lived at Ekachakrapura for six months.

After that they attended Swayamvar of Draupadi.

By that time Arjuna was 28 years old .

So Arjuna faced Karna when he was 28 year old in after Swayamvara battle.